The examination involves the fairness of Google search results, a concept called search neutrality. Some companies worry Google has the power to discriminate against them by lowering their links in search results or charging higher fees for their paid search ads.

In a company blog post, Don Harrison, Google’s deputy general counsel, said that the company’s priority is to “provide the most useful, relevant search results and ads for users.”

“Given that not every Web site can be at the top of the results, or even appear on the first page of our results, it’s unsurprising that some less relevant, lower quality websites will be unhappy with their ranking,” Mr. Harrison wrote.

Lauri Saathoff, a spokeswoman for the Texas attorney general, confirmed the investigation but declined to give more information because the review is ongoing.

The issue is front and center as Google moves into many new business areas, including local business listings, shopping comparison and travel.

The Texas attorney general has asked Google for more information on several companies, Google said. They include Foundem, a British shopping comparison site, SourceTool, a business search directory and myTriggers, which collects shopping links.

In the Google blog post, Mr. Harrison drew an association to Microsoft. He said that Microsoft funds Foundem’s backer and that its antitrust attorneys represent the other two.

Foundem is a member of the Initiative for a Competitive Online Marketplace, a European group co-founded and sponsored by Microsoft. SourceTool and myTriggers are clients of Cadwalader, Wickersham & Taft, the law firm that represents Microsoft on antitrust issues.